1. Field of the Invention
The present invention relates to a transparent conductive film and a method for its production, and a sputtering target to be used for producing the transparent conductive film.
2. Discussion of Background
ITO (indium-tin-oxide), which is indium oxide doped with tin, is well known as a transparent and conductive oxide, and an ITO film is widely used as a transparent conductive film for a solar cell and a liquid crystal display.
On the other hand, recently, a zinc oxide-based transparent conductive film has come to be known as a transparent conductive film comparable to an ITO film in transparency and conductivity, and it is expected to become industrially practical in view of cheapness of a zinc oxide-type target as compared with an ITO-type target.
Aluminum is the most common as a dopant for zinc oxide, but it is still impossible to steadily form a film having a low electrical resistance by using aluminum as a dopant, therefore other dopants are under investigation and have been proposed, for example, in Japanese Examined Patent Publication No. 72011/1991.
In film formation methods using a vacuum apparatus such as a sputtering method, before film formation, a film formation chamber which has been left open to the atmospheric air should be previously evacuated to a high vacuum so that the pressure is lower than the pressure during film formation. In such a case, the atmospheric moisture and the moisture which the walls of the chamber adsorbed during the chamber was left open to the atmospheric air (the moisture is desorbed under vacuum) can not be completely removed by means of a vacuum pump, and part of them remain in a vacuum. The amount of the meaning moisture is influenced by factors which are difficult to control such as the atmospheric humidity when the chamber is left open to the atmospheric air, stains on the walls of the film formation chamber and the conditions of the vacuum pump, therefore, it is virtually difficult to control the amount of the remaining moisture to a constant degree. In formation of a transparent conductive film of a zinc oxide type, because zinc oxide is liable to hydrate, and reacts with such remaining moisture, there has been a problem that electrical characteristics of the resulting film varies depending on the amount of the remaining moisture.
Accordingly, a method for producing transparent conductive films of a zinc oxide type having steady characteristics without controlling the remaining moisture in a vacuum has been desired.
In addition, conventional transparent conductive film of a zinc oxide type do not have moisture resistance enough to be used in a humid atmosphere, therefore the improvement in moisture resistance has been desired.